Thermoelectric conversion materials formed of a filled skutterudite-based alloy have low thermal conductivity, as compared with an intermetallic compound, such as CoSb3, having a skutterudite-type crystal structure, which compound is a type of conventional thermoelectric conversion materials. Therefore, such thermoelectric materials formed of a filled skutterudite-based alloy show promise as thermoelectric conversion materials for use particularly in a high-temperature range.
A filled skutterudite-based alloy is an intermetallic compound represented by the formula RT4Pn12 (wherein R represents a rare earth metal, T a transition metal, and Pn an element, such as P, As or Sb). In the alloy, interstitial spaces present in skutterudite-type crystals represented by formula TPn3 (wherein T represents a transition metal, and Pn an element, such as P, As or Sb), are partially filled with large-mass atoms, such as rare earth metals (R). One reason why thermoelectric conversion materials formed of a filled skutterudite-based alloy have low thermal conductivity is that, when interstitial spaces included in the skutterudite-type crystals are filled with rare earth metal elements, the rare earth metal elements cause vibration, by virtue of weak bonding between the elements and Pn, thereby providing phonon scattering centers.
Appropriate selection of R or T is considered to allow selective conversion of the filled skutterudite-based alloy into either a p-type material or an n-type material. Thus, in order to select the p-type or n-type, attempts have been made to substitute elements, such as Co and Ni, for part of component T comprising Fe atoms.
The thus produced p-type and n-type filled skutterudite-based alloys are shaped into blocks, and a p-type block and an n-type block are directly or indirectly (i.e., by the mediation of a metallic conductor) joined together so as to form a p-n junction, whereby a thermoelectric conversion element can be fabricated. Alternatively, a thermoelectric conversion element module (U- or V-shape) can be fabricated by connecting p-type and n-type filled skutterudite-based alloy thermoelectric conversion members so as to form a p-n junction. As another alternative, a series of thermoelectric conversion elements having a p-n junction are connected and equipped with a heat exchanger to thereby provide a thermoelectric conversion system, through which electricity can be generated on the basis of a difference in temperature.
Conventionally, thermoelectric conversion elements have been fabricated by use of a filled skutterudite-based alloy in such a manner as comprising the steps of weighing high-purity powder materials of a rare earth metal, a transition metal, P, As, Sb, etc. so as to attain the composition of a target filled skutterudite alloy, mixing the materials, calcining the mixture at 800° C. or lower, pulverizing the calcined product, hot-press-sintering or plasma-discharge-sintering the pulverized product by heating to 800° C. and cutting the sintered product.
However, when the above method is employed, the crystal grain size of the formed filled skutterudite-based alloy is greatly affected by the conditions of material powder. In addition, there arises a problem that an increase in crystal grain size, which tends to occur when sintering conditions are not strictly controlled, causes a deteriorated performance of the fabricated thermoelectric conversion elements.
In order to avoid the above problem, there has been proposed a technique where a sintered product of Sb-containing skutterudite-based thermoelectric material, which is a type of filled skutterudite-based thermoelectric conversion material, is formed from minute skutterudite-structure crystal grains and a metal oxide is dispersed in the grain boundaries of the crystal grains (JP-A 2000-252526).
The publication discloses that the use of the above technique reduces the mean crystal grain size of the skutterudite-structure crystal grains to 20 μm or less. However, the method has a problem that the presence of metal oxide in the crystal grain boundaries lowers electric conductivity.
Another method for producing a thermoelectric conversion material formed of a filled skutterudite-based alloy is a heat treatment of ribbons fabricated through the melt-spinning method (JP-A 2002-26400). The melt-spinning method generally includes pouring a molten metal under pressure onto a roller that is rotating at high speed, from a nozzle formed of a quartz-made tube having a hole of approximately 1 mm in its tip.
However, even when the method is employed, a filled skutterudite thermoelectric conversion element having a satisfactory purity is difficult to obtain since the produced alloy ribbons assume amorphous or contain decomposition products, such as Sb2Fe and Sb. Thus, the alloy ribbons must be heated at 873 K to 1,073 K for five hours or longer so as to attain a practically usable purity, thereby constituting another problem.
Furthermore, in any of the aforementioned methods, when steps from a material preparation step to a sintering step are carried out in an oxygen-containing atmosphere, such as air, rare earth metal atoms are removed from the crystal lattice of a filled skutterudite structure by the oxidation of rare earth metals, resulting in partial decomposition of the skutterudite structure to form Sb2Fe and Sb, which is also problematic.
One object of the present invention is to provide a method for producing a filled skutterudite-based thermoelectric conversion material without requiring adoption of an alloy-pulverizing step and a pulverized product-sintering step.
Another object of the invention is to provide a filled skutterudite-based alloy advantageously usable for a thermoelectric conversion element without being modified.
Still another object of the invention is to provide a thermoelectric conversion element fabricated using the above filled skutterudite-based alloy.